Proton beam radiotherapy holds the promise for improving local control of cancer as it permits improved dose localization compared to perhaps any other radiation technique. The improved dose localization, possible through the dose deposition properties of charged particles, permits higher tumor doses with increased sparing of normal tissue doses. Thus, both an increase in tumor control and a reduction in radiation morbidity are expected. Current and future proton beam facilities require instrumentation to monitor the proton beam in real time for control and safety. Current developments in dynamically controlled scanned proton beams are expected to further improve the therapeutic advantage of protons. These developments, and the comparable developments in conventional X-ray radiotherapy, further increase the need for accurate and fast detector instrumentations. We will develop a new scintillator-based detector for the real-time monitoring of a scanned, narrow-focused, proton beam during the irradiation of a patient. This detector will track the position of the proton beam with millimeter and microsecond resolution in real-time in order to verify the relevant spatial and dosimetric beam parameters. The detector can provide feedback into the control system of the scanning beam to dynamically correct for any deviations in the beam parameters. The use of a scintillator minimally affects the proton beam and ensures that delicate instrument components are not unduly exposed to primary or scattered radiation. The only component in the beam, the scintillator, is inherently radiation robust and should show little aging due to radiation exposure, and is inexpensive to replace if needed. In Phase I, we will validate the main detector components and performance, including position response, time response, and accuracy using the proton beam facilities at the Northeast Proton Therapy Center, NPTC, at the Massachusetts General Hospital. In Phase II, we will construct a working detector that can be integrated into the treatment facility at the NTPC, and which can be easily replicated for other proton facilities. A scanning proton beam facility requires a detector system that performs like the one proposed. While many other elements go into a useful proton beam facility, this detector is one essential element of the technology that will promote improved control of cancerous tumors. [unreadable] [unreadable]